The present invention relates to the field of combustion turbines, and, more particularly, to a catalytic combustor for a combustion turbine.
A combustion turbine typically includes three main sections. The first is a compressor that takes in air from the atmosphere and compresses it. The second is a combustor that mixes the compressed air with fuel and ignites the mixture. And the third is a turbine that converts the heat energy resulting from combustion into mechanical energy for powering equipment such as a rotating shaft used to drive an electrical power generator.
Among the by-products of the reactions associated with the combustion are nitrogen oxides (NOx), which are known to contribute to air pollution. Because the combustion turbine releases combustion gases back into the atmosphere, strenuous efforts have been made to reduce the content of nitrogen oxides.
An approach to controlling nitrogen oxide emissions during operation of the combustion turbine uses a catalytic combustor. A catalytic combustor uses a catalyst to facilitate combustion so that it can occur at lower temperatures than those associated with conventional combustors (i.e., about 2500xc2x0 F. versus about 4500xc2x0 F. with conventional combustors). The lower temperatures are typically too low to promote oxidation of nitrogen, and thus the emission of nitrogen oxides can be reduced.
Some catalytic combustors premix fuel and air prior to combustion so that the mixture is relatively lean with respect to fuel (i.e., a lean fuel mixture). During combustion, excess air absorbs heat and accordingly limits the rise in temperature of the products of combustion so that the production of nitrogen oxides is curtailed or prevented.
A problem associated with the use of a lean fuel mixture is that the typical catalyst may not be active at the temperature at which the mixture components leave the compressor (i.e., compressor discharge temperature). A second problem relates to heavy duty combustion turbines in which, even with a lean fuel mixture, the load is so great that the temperature needed for combustion overheats the catalyst as the mixture passes over the catalyst.
One approach to the first problem is to use a preburner, as disclosed, for example, in U.S. Pat. No. 5,850,731 to Beebe et al. The preburner is incorporated into the combustion turbine to heat air prior to its reaching the catalyst. Beebe et al. also addresses the second problem in disclosing a post-catalytic combustion zone. The post-catalytic combustion zone is part of the combustor and is downstream of a catalytic combustion zone. Additional lean fuel and air mixture is supplied to the post-catalytic combustion zone when the combustion turbine is operated at high-load conditions.
Although catalytic combustors using lean fuel mixtures may reduce nitrogen oxide emissions, the additional devices such as preburners often needed to overcome problems associated with the combustion of such a mixture can be costly and add to the complexity of the catalytic combustor. Moreover, with added complexity, there are more opportunities for operational difficulties and breakdowns.
Accordingly, other catalytic combustors use a rich fuel and air mixture. A problem associated with the rich fuel and air mixture, however, is that its combustion leads to greater temperature increases as compared to the lean fuel and air mixture. The increased temperature can damage the catalyst. Thus, while the use of the rich fuel and air mixture overcomes the problems associated with a lean fuel and air mixture, it gives rise to a different problemxe2x80x94namely, that of increased temperatures during catalytic combustion.
One approach is to provide parallel passages, some of which are lined with a catalyst and others of which are not lined. Combustion occurs in the catalyst-lined passages when the fuel and air mixture flows through them, but not in the unlined ones. Thus, the mixture in the unlined passages remains cool and serves to reduce the temperature rise associated with the combustion in the catalyst-lined passages.
Conventionally, the parallel passages are provided by a honeycomb structure as disclosed, for example, in U.S. Pat. No. 4,870,824 to Young et al. and U.S. Pat. No. 4,413,470 to Scheihing et al. Both Young et al. and Scheihing et al. disclose catalytic combustors that comprise a can or housing within which a honeycomb structure is supported.
Young et al., more particularly, describes the honeycomb structure as comprising a plurality of criss-cross intersecting walls defining a series of parallel passages. A catalyst is coated on selected wall surfaces, whereas other wall surfaces remain free of catalyst coating. A mixture passes through the latter passages without reacting to generate heat, but instead providing passive cooling.
Although honeycomb structures can provide desired cooling, their downstream mixing characteristics with respect to the heated gases that are subsequently passed to the turbine are less than desirable. An alternative, therefore, is to provide the similar style cooling using tube arrays. Like honeycomb structures, however, tube arrays are susceptible to vibration-induced stress and fatigue.
In view of the foregoing background, it is therefore an object of the present invention to provide a more robust catalytic combustor having good downstream mixing characteristics.
This and other objects, features, and advantages in accordance with the present invention are provided by a catalytic combustor having a frame that carries a plurality of catalyst support plate assemblies that are less susceptible to vibration-induced stress and that provide effective downstream mixing. The catalyst support plate assemblies carried by the frame may each comprise a pair of opposing plates. At least one of the plates may have ridges thereon to define passageways between the pair of opposing plates. A catalyst may be carried by each of the catalyst support plate assemblies.
In one embodiment, both opposing plates of a catalyst support plate assembly have ridges, and valleys extend between adjacent ridges. Accordingly, a pair of opposing plates may be aligned with respect to one another and be connected at their opposing ridges to define air passageways of a predetermined shape. More particularly, the predetermined shape of the air passageways may be circular. Additionally, the predetermined shape of the air passageways may be flared at an outlet end of the catalytic combustor.
The catalyst support plate assemblies may be arranged in a back-to-back relation so that adjacent pairs of catalyst support plate assemblies define fuel/air passageways therebetween. Adjacent pairs of catalyst support plate assemblies also may be offset from one another to define a nested configuration.
Some or all of the catalyst support plate assemblies may carry a catalyst. Thus, the fuel/air passageways may be lined with the catalyst. More particularly, catalyst material may be coated on the opposing surface of the valleys of one or both opposing plates of a support plate assembly. As catalyst-assisted combustion occurs, cooling air flowing within the air passageways provides cooling.
The plurality of catalyst support plate assemblies may be arranged in a plurality of trapezoidally shaped modules. The modules, in turn, may be arranged to collectively define a generally circular shape with a central passageway extending therethrough.
An additional aspect of the invention relates to a method for making a catalytic combustor. The method may include forming a plurality of plates, at least some of which have ridges with valleys between adjacent ridges. The method also may include assembling the plurality of plates into a plurality of catalyst support plate assemblies such that each catalyst support plate assembly comprises a pair of opposing plates with at least one of the plates having ridges that define air passageways between the opposing plates.
The method may further include arranging the catalyst support plate assemblies in back-to-back relation so that adjacent pairs of catalyst support plate assemblies define fuel/air passageways between the catalyst support plate assemblies. Accordingly, the method also may include coating a catalyst on the catalyst support plate assemblies so that the catalyst lines the fuel/air passageways.